Like Candlemas, Lammas and Halloween, May Day is one of the corner days which fall between the solar festivals of the year (the equinoxes and solstices). The ancient Celts called this holiday Beltane and began celebrating at sunset on April 30th. It marked the beginning of summer, time to move with the flocks up to the summer pastures.
Alexander Carmichael, a 19th century amateur folklorist, describes the annual procession to the summer pastures in language which reminds me of more contemporary summer pleasures, like summre camp, summer vacations, summer cabins:
On the first day of May the people of the crofter townland are up betimes and busy as bees about to swarm. This is the day of migrating, bho baile gu beinn (from townland to moorland), from the winter homestead to the summer sheiling. The summer of their joy is come, the summer of the sheiling, the song, the pipe and the dance, when the people ascend the hill to the clustered bothies, overlooking the distant sea from among the fronded ferns and fragrant heather, where neighbour meets neighbour, and lover meets lover.
Get some more information from The School of the Seasons website.